Systems for determining the position of a radiation-reflecting body have been proposed wherein two fan-shaped beams of radiation, such as laser radiation, were swept flatwise angularly across a solid angle that had the emitter of the radiation at its apex. By reason of the fanwise divergence of each beam in the direction away from the emitter, each beam had a long dimension in cross-section and a transverse narrow one. In most such prior systems each beam had its said long dimension oriented at right angles to that of the other and was swept transversely to that long dimension. Usually the beams were swept alternately in a repetitive sweep cycle. By taking note of the angular position of each beam in its sweep at the instant when a reflection from a body was received at the emitter location, two of the three coordinates that define the position of the body in the space swept by the beams could be known.
It is also readily possible to obtain information about the distance between a radiation emitter and a reflecting body by measuring the time interval between emission of radiation and reception of the reflected radiation at the emitter. That time interval is of course a simple function of the distance to be measured.
Heretofore these theoretical possibilities for determining the location of a reflecting body by means of a pair of fan-shaped, flatwise sweeping beams of radiation could be satisfactorily realized only on the condition that there was no more than one such body in the space swept by the beams. As soon as two or more such bodies were present in that space, they gave rise to a problem of ambiguity to which there was no completely satisfactory solution.
The nature of the ambiguity problem is pointed out in U.S. Pat. Nos. 2,514,617 and 3,056,129, both issued to W. J. Albersheim. The problem can be more fully understood from a consideration of FIG. 1 of the accompanying drawings, wherein fan-shaped beams x and y are shown in cross-section. Those two beams alternately sweep across a space 1, the beam x being swept flatwise horizontally and the beam y being swept flatwise vertically. If reflecting bodies are present in the space at the positions denoted by A and B, then a reflection will be detected at the beam emitter location at the instant when the x beam is at the position of its sweep designated by x.sup.1 and again when it is at the position designated by x.sup.2. Similarly, reflections of the y beam will be received at the instant when it is in its angular positions of sweep that are respectively designated by y.sup.1 and by y.sup.2. These received reflections correspond to four possible positions A, B, C, D at which reflecting bodies may be present in the swept space, and from the available information it is equally as probable that bodies are present at positions C and D as at positions A and B.
In general, such an ambiguity can arise whenever the number of reflectors in a space swept by fan-shaped beams is equal to or greater than the number of beams sweeping the space. The implication of this principle is that ambiguities could be resolved by the employment of a sufficiently large number of discrete beams. But it is obvious that if the presence of numerous reflecting bodies could be expected, it would be difficult to generate a sufficient number of beams, all oriented at different angles, and to synchronize their several sweeps and calculate the measurement results obtained with them.
The two Albersheim patents suggest the employment of so-called range gating by which indications are accepted only from those targets that are at a predetermined distance from the measuring station, or within a predetermined range of distances from that station. Range gating can reduce the number of bodies that must be identified, but it is of no avail when two reflecting bodies are at the same distance from the measuring station and give rise to the ambiguity problem just explained. Albersheim U.S. Pat. No. 2,514,617 proposed an expedient for resolving that ambiguity, but as stated in the later Albersheim patent, the system of that earlier patent required the performance of a number of time consuming operational steps and was therefore unsatisfactory in applications where time was of the essence. The later Albersheim patent proposed an expedient which required more complicated and costly apparatus and which, although faster, was still rather slow in that it required the performance of several calculating operations for the purpose of obtaining information about actual reflector locations. There is also reason to doubt that the system of the later Albersheim patent could produce unambiguous results under all circumstances.
Those skilled in this field of art will recognize that there are a variety of applications in which fan-shaped flatwise sweeping beams could be used for complete measurement of reflecting body locations, given a solution to the problem of unambiguous identification of individual bodies when plural bodies appear in the space swept by the beams and particularly when two or more bodies in that space are at the same distance from the measuring station. As examples of such applications, mention can be made of systems for supervision and control of taxiing aircraft on an airport, measurement of air or water currents by tracking of balloons or floats that move with the current, and continuous measurement of positions of boats in a sailing competition.
An application which is of particular concern, and which particularly exemplifies the utility of the present invention, is that of measurement of target location in simulated weapons fire scoring systems. One such system is disclosed in U.S. Pat. No. 3,832,791, wherein a beam having a substantial amount of divergence was emitted from the weapon location so that any target at which the weapon was aimed could reflect radiation back to the weapon location, notwithstanding gun elevation and aiming lead to compensate for target movement. A first emission of the beam, occuring at the instant of simulated firing, was employed to obtain a ranging fix on the target; and after an interval equal to the calculated time required for a round of ammunition to arrive at the target, a second emission of the beam was modulated to encode information about ammunition type and point of impact of the simulated round in relation to the target, so that hit effect could be evaluated at the target. Because of the divergence of the beam, it was necessary to have a specialized and relatively expensive detector that could distinguish between reflections from target reflectors that were at the same range and relatively close to one another, and transmissions could not be made on the beam exclusively to a detector co-located with a particular one of such reflectors. Therefore there were many tactical situations in which accurate scoring results could not be obtained. As a further result of the divergence of the beam and the consequent diffusion of its radiation, the system had relatively poor range for a given amount of radiation energy and had a poor ratio of signal to background disturbance.
By contrast, the general object of the present invention is to provide a measuring system wherein beamed radiation is employed for accurate determination of the position of a reflecting target, which system is better suited to simulated weapons fire scoring applications than prior such systems, and is also very advantageously applicable to many other types of equipment for remote measurement of positions of objects.
Application of the present invention to a system for scoring simulated gunnery practice requires the solution of certain further problems, and with respect to these the present disclosure is supplemented by the disclosures of two copending applications, Ser. No. 14,115 and Ser. No. 14,116.
Ser. No. 14,116 relates to a method and means for effecting selective delivery of information encoded in the modulations of flatwise sweeping fan-shaped beams, so that information intended only for one of a plurality of bodies in the space swept by the beams will be delivered exclusively to that one body.
The other copending application, Ser. No. 14,115 is somewhat more closely related to the subject matter of the present invention, inasmuch as it discloses a gunnery practice scoring system which, in one of its operating modes, employs periodically sweeping fan-shaped beams for making measurements of target position from and after the instant of simulated firing of a weapon, and simultaneously makes a calculation of the position of an imaginary projectile in its trajectory. The trajectory calculation simulates the flight that a selected type of real projectile would have had if fired from the weapon with its barrel axis oriented as at the instant of simulated firing. At the instant when the calculated position of the imaginary projectile brings it to a distance from the weapon location that is equal to the weapon-to-target distance, or when the imaginary projectile reaches a predetermined elevation relative to target elevation, the calculation can stop and results can be scored at the weapon location on the basis of the relationship between projectile position and target position at that instant.
In such a scoring system, each target comprises a reflector by which intercepted radiation emitted from the weapon location is reflected back to that location. The beams are pulsed so that reflected radiation, detected at the weapon location, can be employed there for measuring distance to the reflector. Target reflector azimuth and elevation relative to the weapon location are measured by taking account of the momentary angular position of each beam in its sweep at the time its radiation, reflected from the reflector, is detected at the weapon location.
It will be apparent that accurate and realistic utilization of such a scoring system requires that when reflections are received during the course of a beam sweep from two or more target reflectors that are at equal distances from the weapon position, the scoring apparatus shall not respond to spurious target positions created by the above described ambiguity problem and should be capable of discriminating between adjacent reflectors. This requirement is imposed because there are many tactical situations--for example, tank maneuvers--in which two or more target reflectors may be present in proximity to one another and at substantially equal distances from a weapon that has them in its field of fire, and such tactical situations should be reproduced during training exercises in the interests of training effectiveness. Obviously range gating would not be satisfactory for unambiguous determination of the positions of the targets in such a situation.
With the foregoing considerations in mind, the general object of the present invention can be more fully stated as being to provide a method and means for unambiguously ascertaining at a measuring station, by means of fan-shaped, flatwise sweeping beams emitted therefrom, the position relative to said station of each of a plurality of reflectors that may be present in a solid angle which is swept by the beams and which has the station at its apex.
A more specific object of the invention is to provide a measuring system wherein fan-shaped beams of radiation are emitted from a measuring station and are swept flatwise angularly across a solid angle space that has the measuring station at its apex, wherein said beams are employed to ascertain the position within said space of a reflector of beam radiation that is at a distance from the measuring station, and whereby an unambiguous determination can be made of the position of each of a plurality of such reflectors that may be present in said space.
Another specific object of this invention, and a very important one, is to provide a method and means for rapidly and efficiently resolving the ambiguity that has heretofore arisen with the use of flatwise-sweeping fan-shaped beams used for measurement of the positions of reflectors in the space swept by the beams when the number of reflectors in that space was equal to or greater than the number of beams employed.
Other objects of the invention include the provision of a fast method of measurement wherein radiation emitted from a measuring station is employed to obtain an unambiguous measurement of the position of each of a group of reflectors in an area remote from the measuring station, or of the position of a selected one of such reflectors, and whereby such position measurements can be obtained as functions of range, azimuth and elevation relative to the measuring station, which method can be practiced with simple automatic equipment, ensures good range and high sensitivity by reason of low radiation diffusion, and affords the capability for discriminating between reflectors that are relatively close to one another.